The Battle of the Frontiers (French: Bataille des Frontières; German: Grenzschlachten; Dutch: Slag der Grenzen) comprised battles fought along the eastern frontier of France and in southern Belgium, shortly after the outbreak of the First World War. The battles resolved the military strategies of the French Chief of Staff General Joseph Joffre with Plan XVII and an offensive adaptation of the German Aufmarsch II deployment plan by Helmuth von Moltke the Younger. The German concentration on the right (northern) flank, was to wheel through Belgium and attack the French in the rear.
The German advance was delayed by the movement of the French Fifth Army under General Charles Lanrezac towards the north-west to intercept them and the presence of the British Expeditionary Force (BEF) on the French left. The Franco-British troops were driven back by the Germans, who were able to invade into northern France. French and British rearguard actions delayed the Germans, allowing the French time to transfer forces on the eastern frontier to the west to defend Paris, culminating in the First Battle of the Marne.
Background
Belgian plans
Belgian military planning assumed that other powers would fulfil their obligations under the Treaty of London (1839), to assist the Belgian Army to eject an invader. The British and French had not formed a formal alliance despite the Anglo-French Entente (1904). The Belgians judged that the British attitude towards their country had changed and that Belgium had come to be seen as a British protectorate. A Belgian General Staff was formed in 1910 but the Chef d'État-Major Général de l'Armée, Lieutenant-Général Harry Jungbluth was retired on 30 June 1912 and not replaced by Lieutenant-General Chevalier de Selliers de Moranville until May 1914. Moranville began planning for the concentration of the army and met Belgian railway officials on 29 July.
The Belgian army was to be massed in central Belgium, in front of the National redoubt of Belgium, ready to face any border, while the Fortified Position of Liège and Fortified Position of Namur were left to secure the frontiers. On mobilisation, the King became Commander-in-Chief and chose where the army was to concentrate. Amid the disruption of the new rearmament plan, disorganised and poorly trained Belgian soldiers would benefit from a central position to delay contact with an invader but it would also need fortifications for defence, which were on the frontier. A school of thought wanted a return to a frontier deployment, in line with French theories of the offensive. Belgian plans became a compromise, in which the field army concentrated behind the Gete river, with two divisions further forward at Liège and Namur.
Schlieffen–Moltke Plan
Alfred von Schlieffen, Chief of the Imperial German General Staff (Oberste Heeresleitung, OHL) from 1891–1906, devised plans for a decisive battle against the French army in Germany, Belgium or France. Aufmarsch I West was a contingency plan for a Franco-German war, in which France (due to fewer numbers) would be on the defensive and Germany would attack by invading Belgium between Antwerp and Namur to advance south and breach the Verdun–Marne–Paris defensive area. The German armies would then pause until railways could be repaired and supplies accumulated for a second offensive operation. Helmuth von Moltke the Younger succeeded Schlieffen in 1906 and became convinced that an isolated Franco-German war was impossible and that Italian and Austro-Hungarian forces would not be available to defend the Franco-German border as had been planned. Aufmarsch I was abolished but in 1914 Moltke tried to apply the offensive strategy of Aufmarsch I to the deployment plan Aufmarsch II for a two-front war. In 2014, Terry Holmes wrote,
From his assessment of French defensive capability Schlieffen concluded that the German army would need at least 48.5 corps to succeed with an attack on France by way of Belgium, but Moltke planned to attack through Belgium with just 34 corps at his disposal in the west. The Schlieffen plan [sic] amounts to a critique of German strategy in 1914 since it clearly predicted the failure of Moltke’s underpowered invasion of France. [...] Moltke followed the trajectory of the Schlieffen plan, but only up to the point where it was painfully obvious that he would have needed the army of the Schlieffen plan to proceed any further along these lines.
The main German force tried to follow Aufmarsch I to envelop the French armies on the left (north) and press them back over the Meuse, Aisne, Somme, Oise, Marne and Seine rivers, unable to withdraw into central France. Moltke hoped that the French would either be annihilated or the manoeuvre from the north would create conditions for victory in the centre or in Lorraine, on the common border.
Plan XVII
Under Plan XVII, the French peacetime army was to form five field armies of c. 2,000,000 men, with groups of Reserve divisions attached to each army and with a group of reserve divisions on the southern and northern flanks. The armies were to concentrate opposite the German frontier around Épinal, Nancy and Verdun–Mezières, with an army in reserve around Ste Menehould and Commercy. Since 1871, railway building had given the French General Staff sixteen lines to the German frontier against thirteen available to the German army; the French could wait until German intentions were clear. The French deployment was intended to be ready for a German offensive in Lorraine or through Belgium. It was anticipated that the Germans would use reserve troops but also that a large German army would be mobilised on the border with Russia, leaving the western army with sufficient troops only to advance through Belgium south of the Meuse and the Sambre rivers. French intelligence had obtained a map exercise of the German general staff of 1905, in which German troops had gone no further north than Namur and assumed that plans to besiege Belgian forts were a defensive measure against the Belgian army.
A German attack from south-eastern Belgium towards Mézières and a possible offensive from Lorraine towards Verdun, Nancy and St Dié was anticipated. The plan was an evolution of Plan XVI and made more provision for the possibility of a German offensive through Belgium. The First, Second and Third armies were to concentrate between Épinal and Verdun opposite Alsace and Lorraine, the Fifth Army was to assemble from Montmédy to Sedan and Mézières. The Fourth Army was to be held back, west of Verdun, ready to move east to attack the southern flank of a German invasion through Belgium or southwards against the northern flank of an attack through Lorraine. No formal provision was made for combined operations with the British Expeditionary Force (BEF) but joint arrangements had been made and in 1911, during the Second Moroccan Crisis, the French had been told that six divisions could be expected to operate around Maubeuge.
Declarations of war
At midnight on 31 July/1 August, the German government sent an ultimatum to Russia and announced a state of Kriegsgefahr during the day; the Ottoman government ordered mobilisation and the London Stock Exchange closed. On 1 August, the British government ordered the mobilisation of the navy, the German government ordered general mobilisation and declared war on Russia. Hostilities commenced on the frontier of Congress Poland; the French government ordered mobilisation and next day the German government sent an ultimatum to Belgium demanding free passage, as German troops were crossing the frontier of Luxembourg. Military operations began on the French frontier, Libau was bombarded by a German light cruiser SMS Augsburg and the British government guaranteed naval protection for French coasts. On 3 August, the Belgian Government refused German demands and the British Government promised military support to Belgium should Germany invade. Germany declared war on France, the British government ordered general mobilisation and Italy declared neutrality. On 4 August, the British government sent an ultimatum to Germany and declared war at midnight on 4/5 August, Central European Time. Belgium severed diplomatic relations with Germany and Germany declared war on Belgium. German troops crossed the Belgian frontier and attacked Liège.
Prelude
French preparations
Joseph Joffre, who had been Commander-in-Chief of the French army since 1911 and the Minister of War, Adolphe Messimy met on 1 August, to agree that the military conduct of the war should exclusively be the responsibility of the Commander-in-Chief. On 2 August, as small parties of German soldiers crossed the French border, Messimy told Joffre that he had the freedom to order French troops across the German but not the Belgian frontier. Joffre sent warning orders to the covering forces near the frontier, requiring the VII Corps to prepare to advance towards Mühlhausen (Mulhouse) to the north-east of Belfort and XX Corps to make ready to begin an offensive towards Nancy. As soon as news arrived that German troops had entered Luxembourg, the Fourth Army was ordered to move between the Third and Fifth armies, ready to attack to the north of Verdun. Operations into Belgium were forbidden, to deny the Germans a pretext until 4 August, when it was certain that German troops had already violated the Belgian border. To comply with the Franco-Russian Alliance, Joffre ordered an invasion of Alsace-Lorraine for 14 August, although anticipating a German offensive through Belgium.
On 8 August, General Instruction No. 1 had ordered the Fifth Army to take a position on the left of the Fourth Army, ready to attack the southern flank of German force advancing from Mézières and Mouzon, through the difficult terrain in between. All four corps covered this front until 12 August, when Joffre allowed General Charles Lanrezac to move I Corps north to Givet to oppose a potential German attempt to cross the Meuse between Givet and Namur 35 km (22 mi) further north, which extended the army front to 80 km (50 mi). As Lanrezac became aware of the size of the German force in Belgium and wanted to reinforce the left flank by moving to Namur, Joffre refused to allow the army front to be extended to 110 km (68 mi) and ordered Lanrezac to keep the army in a central position near Mézières, ready to oppose a German offensive from Mouzon to Namur. On 14 August Joffre and Lanrezac met but Joffre considered that only a few German cavalry and infantry parties had crossed the Meuse. With the BEF moving to Maubeuge and Hirson a redeployment of the Fifth Army would disrupt the deployment of the other armies. On 14 August a new intelligence report showed eight German corps between Luxembourg and Liège and by the next day Joffre allowed the move of the Fifth Army north, to operate beyond the Meuse. The XI Corps was transferred to the Fourth Army and the XVIII Corps was moved from the Third Army to the Fifth Army, which was made responsible for the defence of Maubeuge.
Joffre began to dismiss commanders in early August, beginning with the VII Corps commander, Major-General Louis Bonneau; by 6 September he had removed two army, ten corps and 38 divisional commanders, by transferring them to Limoges (Limogé). The VII Corps in the south was reinforced by two divisions, a cavalry division and the First Group of Reserve Divisions. The corps was renamed the Army of Alsace, to relieve the First Army of concern about Alsace during the operations in Lorraine. Two corps were removed from the Second Army and became a strategic reserve. Joffre met Sir John French on 16 August and learned that the British could be ready by 24 August, Joffre also arranged for Territorial divisions to cover the area from Maubeuge to Dunkirk. The German siege of the Liège forts ended on 16 August and the 1st and 2nd armies with twelve corps and the 3rd Army with four corps began to advance behind cavalry screens. On 18 August, Joffre ordered the Fifth Army to prepare for a German offensive on both banks of the Meuse or to meet a small force on the north bank. The Fifth Army began to move towards Namur, in the angle of the Meuse and Sambre rivers on 19 August, which required a march of 100 km (62 mi) by some units.